Feat/headless canvas

[myers]

Hello! 您好

This builds on my work in https://github.com/alibaba/canvas-ui/pull/34, I suggest you will want to look at the last commit to understand this PR. When that other PRs are merged I will update this PR and remove the draft designation.

This PR adds HeadlessCanvas, a React component for rendering Canvas UI to OffscreenCanvas without mounting to the DOM.

Changes:

• Add HeadlessCanvas component with event injection API
• Accept user-provided OffscreenCanvas and custom frameScheduler
• Provide injectEvent and injectWheelEvent helpers via onReady callback
• Support onFrameEnd callback for texture copying
• Export HeadlessCanvas and related types from @canvas-ui/react

Benefits:

• Enables WebXR layer rendering (XRWebGLLayer texture targets)
• Manual frame control with custom schedulers
• Programmatic event injection for XR controllers
• No DOM coupling for headless environments

Use cases:

• WebXR UI panels rendered to quad layers
• Custom render loop integration